It is generally recognized that it is important to be able to determine the body density and, therefore, body volume of objects such as human adults or newborn infants, especially those whose growth and development must be carefully monitored. Clearly, such determinations, while essential, should involve techniques which are as accurate, non-invasive, least disturbing, and efficiently as performed as possible.
Previously known techniques for measuring body volume which involve underwater or hydrostatic weighing and water displacement, for example, are undesirable because of the inherent trauma caused to the person, especially when it is an infant.
Other non-invasive means of measuring body volume are based on containing the object in an airtight chamber, altering the pressure or volume in varying degrees of sophistication and applying thermodynamic principles to deduce the body volume. Generally, long times (up to an hour) requiring repetitive measurements (with empirical correction factors) are needed to establish adequate repetitiveness and accuracy using these techniques.
The inherent difficulties of air displacement devices are: large volume of an air tight container relative to body volume required to prevent asphyxiation; and the thermodynamic process is not simply isothermal or adiabatic, particularly when measurement time is long compared to characteristic times involved in heat transfer and gas exchange as in infant breathing.
These known air displacement volume measuring devices are therefore complex and used only for research and are not likely to become generally available for routine use.
Other systems have employed the principle of the Helmholtz resonator to determine the volume of gas in a container. These systems have however depended on the speed of sound remaining constant between the time that the resonant frequency of the empty chamber is determined and the time that the resonant frequency is determined for the chamber containing the object of unknown volume. Temperature and humidity variations, however, effect the consistency of the speed of sound and therefore the accuracy of techniques which depend on this constancy. Variations in temperature and humidity, for example, are especially pronounced when the object of unknown volume itself produces heat and humidity, such as a living animal.
It is, therefore, an object of the present invention to provide an improved device and method for the quick, easy and inexpensive non-invasive determination of volume of an object which overcomes disadvantages of the prior art and in which variations in temperature, humidity and other factors affecting frequency of sound waves are compensated out.
It is a further object of the present invention to provide such a device and method in which determinations of volume can be accurately made in a single procedure and without the need for insuring isothermal conditions and conditions of constant humidity.